1. Field of the Invention
The present invention relates generally to a receiving apparatus and method for a base station (BS) in an Orthogonal Frequency Division Multiplexing (OFDM)-based broadband mobile communication system, and more particularly, to an apparatus and method for receiving a ranging signal in an Orthogonal Frequency Division Multiple Access (OFDMA) communication system.
2. Description of the Related Art
In a communication system which is defined by an Institute of Electronics and Electrical Engineers (IEEE) 802.16d/e standard, a BS acquires uplink timing synchronization and tracks Carrier-to-Interference plus Noise Ratio (CINR) using a known signal (e.g. a ranging signal, a preamble, a pilot signal, etc.) received from a subscriber station (SS). A signal that the SS transmits to help the BS to acquire the uplink timing synchronization is known as a “ranging signal”. Conventional ranging signal reception will now be described, according to the IEEE 802.16d/e standard.
FIG. 1 is a block diagram schematically illustrating the configuration of an OFDMA-based broadband mobile communication system. The OFDMA communication system is configured to have a single cell structure, and includes a BS 100 and a plurality of SSs 110, 120 and 130 managed by the BS 100. Signal transmission/reception takes place using an OFDM/OFDMA based communication scheme between the BS 100 and the SSs 110, 120 and 130. Thus, the SSs 110, 120 and 130 and the BS 100 transmit physical channel signals on subcarriers.
OFDMA defines an access scheme of a two-dimensional grid that combines Time Division Access (TDM) with Frequency Division Access (FDM). In OFDMA, data symbols are delivered on subcarriers which form subchannels. Depending on system situation, a predetermined number of subcarriers form one subchannel.
For application of Time Division Duplexing (TDD) to the OFDMA communication system, ranging is required to acquire accurate timing synchronization between the SS and the BS and adjust the reception power of the BS on the uplink. In each OFDMA frame a ranging channel has a plurality of subchannels for transmitting a ranging signal.
Ranging in the IEEE 802.16d/e communication system will be described below. The ranging is classified into initial ranging for acquiring physical layer timing synchronization and periodic ranging for maintenance and management.
The initial ranging is the process of acquiring a correct timing offset between the BS and the SS and initially adjusting a transmit power. Upon power-on, the SS acquires downlink synchronization from a received downlink preamble signal. Then the SS performs the initial ranging with the BS to adjust an uplink time offset and transmit power. The IEEE 802.16d/e communication systems use the OFDM/OFDMA communication scheme. Thus, they perform a ranging procedure by transmitting a randomly selected ranging code on a plurality of subchannels.
The periodic ranging is the process of periodically tracking the uplink timing offset and received signal strength after the initial ranging. The SS randomly selects one of ranging codes allocated for the periodic ranging in the ranging procedure.
A description of transmitting a ranging signal will now be provided.
FIG. 2 is a block diagram illustrating a ranging code generator used in a typical TDD/OFDMA system. A Pseudorandom Noise (PN) code generated from a Pseudo Random Binary Sequence (PRBS) generator is used as a ranging code. The generator polynomial for generating a PN code is given asG(x)=1+x1+x4+x7+x15  Equation 1
A register is initialized to 00101011 (binary) and a 7-bit cell identification (ID) number. The SS acquires the cell ID number from a downlink preamble signal or broadcast information.
For a ranging code length of N bits, codes are generated for each ranging mode as follows.
A long sequence is generated under synchronization of 1360th through (N×K1)th clock pulses from the PRBS generator. The long sequence is divided into K1 N-bit codes for use in initial ranging. For handoff ranging, a long sequence generated under synchronization of (N×K1+1)th through N×(K1+K2)th clock pulses from the PRBS generator is divided into K2 N-bit codes. K3 N-bit codes are used for periodic ranging, which are created by dividing a long sequence generated under synchronization of N×(K1+K2+1)th through N×(K1+K2+K3)th clock pulses from the PRBS generator by N bits. For bandwidth request ranging, a long sequence generated under synchronization of (N×K1+K2+K3+1)th through N×(K1+K2+K3+K4)th clock pulses from the PRBS generator is divided into K4 N-bit codes. (K1, K2, K3 and K4 are number of codes).
FIG. 3 is a block diagram illustrating a ranging transmitter in an SS in a conventional TDD/OFDMA communication system.
Referring to FIG. 3, upon receipt of information about an SS-intended ranging mode (e.g. initial ranging, periodic ranging, etc.), a ranging code generator 301 generates a randomly selected ranging code. A ranging channel generator 302 allocates the ranging code to subcarriers. The subcarrier allocation amounts to providing each element or bit of the ranging code to a corresponding input (subcarrier position) of anInverse Fast Fourier Transform (IFFT) processor 303. 0s are padded at subcarrier positions to which the ranging code is not allocated. The IFFT processor 303 generates time-domain signals by IFFT-processing the signal from the ranging channel generator 302. A parallel-to-serial (P/S) converter 304 converts the parallel time-domain signals to serial data. A Cyclic Prefix (CP) inserter 305 inserts a CP into the data stream, thereby creating a baseband ranging signal. While not shown, the baseband ranging signal is processed into a transmittable Radio Frequency (RF) signal and wirelessly transmitted through an antenna.
A ranging channel pattern as defined by the IEEE 802.16e is illustrated in FIG. 4 in which a total of 144 tones (subcarriers) used for transmission of the ranging signal reside in six bands that are separated from each other, each band including 24 successive subcarriers.
Reception of the ranging signal will be described below.
FIG. 5 is a block diagram illustrating a ranging receiver in a BS in the conventional TDD/OFDMA communication system.
Referring to FIG. 5, a Fast Fourier Transform (FFT) processor 501 FFT-processes an input signal and outputs the resulting frequency-domain signal. That is, the FFT processor 501 demodulates the input signal to subcarrier values. A ranging subchannel extractor 502 extracts subcarrier values with a ranging code loaded thereon from the subcarrier values received from the FFT processor 501. A multiplier 503 multiplies the extracted subcarrier values by ranging code 0 (or Code 0). A multiplier 504 multiplies the extracted subcarrier values by ranging code 1 (Code 1). Similarly, a multiplier 505 multiplies the extracted subcarrier values by ranging code (k−1) (Code (k−1)). Without knowledge of a received ranging code, all possible ranging codes are multiplied by the subcarrier values with the ranging code.
A phase detector 506 detects a timing offset from the product received from the multiplier 503. A phase detector 507 detects a timing offset from the product received from the multiplier 504. Similarly, a phase detector 508 detects a timing offset from the product received from the multiplier 505. The operations of the phase detectors 506 to 508 are modeled as defined by Equation 2 below.
                                        ⁢                      (            n            )                          =                  arg          ⁢                                          ⁢                                    max                                                                    t                    min                                    /                                      θ                    step                                                  ≤                n                ≤                                                      t                    max                                    /                                      θ                    step                                                                        ⁢                                          ∑                                                                                                                              m                          ∈                                                      {                                                          0                              ,                              M                                                        }                                                                          ,                                                  RNG                          subband                                                                                                                                                                                                  k                          ∈                                                      {                                                          0                              ,                                                              K                                -                                1                                                                                      }                                                                          ,                                                                              tone                            ⁢                                                                                                                  ⁢                            index                                                    -                          in                          -                          subband                                                                                                                                ⁢                                                Y                                      m                    ,                    k                                                  ⁢                                  C                                      m                    ,                    k                                                  ⁢                                  ⅇ                                                            -                      j                                        ⁢                                                                                  ⁢                    2                    ⁢                    π                    ⁢                                                                                  ⁢                                          f                      ⁡                                              (                                                  m                          ,                          k                                                )                                                              ×                                                                  (                                                  n                          ⁢                                                                                                          ⁢                                                      θ                            step                                                                          )                                            /                                              N                                                  F                          ⁢                                                                                                          ⁢                          F                          ⁢                                                                                                          ⁢                          T                                                                                                                                                                            Equation        ⁢                                  ⁢        2            where Ym,k denotes the received signal response of a kth subcarrier in an mth band in FIG. 4, Cm,k denotes a ranging code bit allocated to the kth subcarrier in the mth band, f(m,k) denotes the frequency index of the kth subcarrier in the mth band, NFFT denotes an FFT size (for example 1024), and θstep denotes samples normalized to a step size (expressed in the number of samples normalized to a sampling rate) set for timing offset detection.
In Equation. 2, {Ym,k, Cm,k,} is the product of the FFT processor output by a ranging code, input to a phase detector. This value is multiplied by an exponential function. A variable set in the exponential function is n and n ranges [tmin/θstep□ tmax/θstep]. n denotes a timing offset range to be estimated. Using Equation 2, {(n), tmin/θstep≦n≦tmax/θstep} is computed over all possible values of n. An n value that maximizes |(n)| is selected as a temporary timing offset, nest.
Peak detectors 509 to 511 each calculate a Peak-to-Average Power Ratio (PAPR) to verify the temporary timing offset received from a corresponding phase detector and compare the PAPR with a predetermined threshold. If the PAPR is greater than the threshold, the temporary timing offset is decided as a timing offset estimate. If the PAPR is less than the threshold, the temporary timing offset is discarded and it is determined that a ranging signal has not been received.
The PAPR is computed using Equation 3 below.
                              P          ⁢                                          ⁢          A          ⁢                                          ⁢          P          ⁢                                          ⁢          R                =                                                                                          ⁢                                  (                                      n                    est                                    )                                                                    2                                average            ⁢                          {                                                                                                                                    ⁢                                              (                        n                        )                                                                                                  2                                ,                                                                            t                      min                                        /                                          θ                      step                                                        ≤                  n                  ≤                                                            t                      max                                        /                                          θ                      step                                                                                  }                                                          Equation        ⁢                                  ⁢        3            
As described above, the conventional TDD/OFDMA communication system detects a ranging signal in the manner illustrated in FIG. 5, and suffers from the following problems.
(1) Acutal implementation is difficult because of computational complexity.
The FFT processor 501 and the multipliers 503 to 505 are basic computation blocks and the phase detectors 506 to 508 detect phases using Equation 2. As noted from Equation 2, 1024 exponential calculations are performed on a value received from a multiplier for one n value and accumulated. Then a maximum value is selected as a temporary timing offset. The peak detectors 509 to 511 calculate PAPRs to verify the temporary timing offsets. The implementation complexity is illustrated in Table 1 below.
TABLE 1FFTreceptionReal(Radi × 2CodeTotalmultiplicationFFT)MultiplicationPhase TestPeak TestcomputationConventionalNFFTlog2NFFT2 × Number_of_Codes ×2 × Number_of_Codes ×2 × Number_of_Codes ×9.46E6Code_SizeCode_Size × NFFTCode_SizeIn Table 3 it is assumed that:NFFT: FFT size (e.g., 1024)Number_of_Codes: the number of ranging codes (e.g., 32)Code_Size: the length of ranging codes (e.g., 144).
As illustrated in Table 1, according to the IEEE 802.16e, 3(ranging type)×9.46E6(computation volume)=28.4E6 real multiplications occur every 5 msec, or 5679E6 floating point calculations take place every second. Therefore, the conventional ranging detection is very difficult to implement.
(2) Ranging reception performance decreases at low Carrier-to-Interference plus Noise Ratio (CINR). Since the ranging channel is not transmitted over the total frequency band, the timing offset estimation can be incorrect.
To be more specific, conventionally, the response of a channel whose phase is rotated by a timing offset in the frequency domain is achieved and then converted to a time-domain channel response, thereby detecting the shift of the time-domain channel response. As described earlier with reference to FIG. 4, since the ranging code is loaded only in some bands, the frequency characteristic of an acquired channel is limited. Meanwhile, conversion of a channel value to the time domain is equivalent to passing through a filter configured in correspondence with a ranging subchannel. Therefore, the output of the phase detector is the convolution of the time response of an ideal channel with a filter coefficient. That is, the phase detector outputs an incorrect timing offset. Considering the effects of noise, the performance is worsened. In a cellular system, many terminals must operate at a low CINR due to inter-cell interference. Since the CINR is a function of distance in constant transmit power and the same path loss, abnormal ranging reception at a low CINR reduces cell radius.